The invention relates to a method for describing a predetermined or desired course with a beam of particles or waves, wherein the direction of the beam can be modified by a movable beam-directing device. The invention also relates to the use of the novel method.
U.S. Pat. No. 5,593,606 discloses methods in which a pulsed laser beam is deflected by a motor-controlled mirror. The data of a predetermined desired course is translated by a drive device into control instructions for the motor. The movable beam-directing device influences the direction of the beam which describes an actual course with a velocity VB when projected onto a surface.
These known methods have the disadvantage that in the case of a desired straight-line course the actual course is described with a constant delay by the response error time tS, owing to the mechanical inertia of the beam-directing device. The effect of this response error is that the actual course that corresponds to a curved desired course deviates therefrom. In this case, the deviation depends on the course velocity VB. FIGS. 1a to 1d show various examples of these deviations. The desired course 1 and the actual course 3 for a low course velocity are represented in each case. An actual course 4 for a high course velocity is also represented in FIGS. 1b and 1c. 
The deviation from the desired course is commensurately greater as the course velocity with which the desired course is described increases. The effect of the deviation of the actual course from the desired course is that, for a constant laser-pulse frequency, an increased number of laser pulses are delivered onto a shortened course segment. This is represented in FIGS. 2a and 2b. FIG. 2a represents a series of laser spots 7 along a desired course 1. The arrows indicate the movement direction of the laser beam. FIG. 2b represents a series of laser spots 7 along the actual course. The arrows indicate the movement direction of the laser beam. If the laser beam is being used to structure a surface, then this increased pulse density can cause higher energy input per area with all possible undesired consequences.
It is an object of the present invention to provide a method for describing a predetermined or desired course with a beam of particles or waves, in which the deviation of the actual course from the predetermined course is minimized. Accordingly, the present invention provides a method for describing a predetermined/desired course with a beam of particles or waves, in which a movable beam-directing device influences the direction of the beam. The beam is directed by the beam-directing device onto a surface, on which it describes an actual course with the velocity VB. A drive device converts the desired course into a corrected desired course, which describes a diversion in the vicinity of a discrete direction change from a first direction of the desired course to a second direction of the desired course. This diversion is obtained by appending and/or deleting arbitrary sections to/from the desired course. The data of the corrected desired course is translated by the drive device into control instructions for the beam-directing device.
Since the desired course to be described is already known, the novel method of correction of the desired course minimizes the deviations of the actual course from the desired course. Direction changes of the desired course where the desired course has corners are particularly critical for the deviations. In the vicinity of these comers, the actual course can be brought closer to the desired course by incorporating into the desired course diversions which extend outside the comers and compensate substantially for the undesired roundings of the actual course which are represented in FIG. 1. Correction sections which may be straight-line sections are particularly useful, because straight desired courses produce no deviations in the actual course.
In methods for describing a predetermined desired course, in which the actual course is specified with a constant course velocity VB, a straight-line desired course is described as the actual course by the laser beam with a constant delay by the response error time tS, owing to the mechanical inertia of the beam-directing device. The method according to the invention uses the response error time tS as a basis and hence is particularly advantageous. The corrected desired course is obtained from the desired course by adding a first straight-line section of length L, which points in the first direction, at the point of a direction change from a first to a second direction. For the length of this straight-line section: L=VBxc3x97tS. Also, a second straight-line section of length L is added at the far end of the first straight-line section from the point of the direction change. The maximum possible course velocity vmax counter to the first direction is specified for this second straight-line section. In this case: vmax greater than vB. The desired course is accordingly extended for a short distance beyond the point of the direction change. Fast return to the point of the direction change thereupon takes place.
The method according to the present invention can be used particularly advantageously for desired courses, which are curved. In this case, the desired course is approximated with a continuous direction change by a desired course with successive discrete direction changes. The desired course is then corrected according to the method of the present invention.
In a preferred embodiment of the invention a pulsed beam is used. During the structuring of a surface by means of laser beams, a problem that may arise is that an increased laser-pulse density is applied to a shortened actual-course segment owing to the course inaccuracies. This increased laser-pulse density can be avoided by the course correction.
The method according to the present invention is preferably used whenever a laser beam is used as the beam, and a mirror with a galvanometer motor is used as the movable beam-directing device. Indeed, for mirrors with a galvanometer motor, the problem the beam-directing device""s mechanical inertia has to date been very noticeable.
The method according to the invention can be used particularly advantageously for processing or modifying a surface, in particular for writing dial plates, or for applying or removing material, for example during laser structuring or doping of semiconductor surfaces by means of ion beams.
Where an ion beam is used as the beam, then it is particularly preferred to use a magnetic lens as the beam-directing device.